For an understanding how a pipette device is assembled and operated, it is referred to German application 25 49 477 and European application 88 11 755 3.3-2113/0 364 621.
The German application referred to discloses an improvement of known pipette devices to facilitate the handling and to reduce the force which is necessary to operate the device. The piston of this pipette is solid and shaped as a rod which tightly enters a cylinder which opens towards an adapter cone on which a pipette tip is placed. The rod shaped piston is suited to take up very small volume, the piston diameter being in the range of 1 to 5 mm.
The European application improves the calibration of a pipette and discloses an embodiment in which the piston is connected to a piston rod through a connecting member which is shaped as a rod of a small cross section. The connecting member allows to make the rod and the piston of different materials.
Contrarily, the present invention refers to a pipette device comprising a piston of a cross section which is larger than the tubular passage to the adapter cone.
This structure facilitates handling varying pipetting volume in providing an individual cylinder chamber above the tubular passage to the adapter cone.
The piston is adjustably mounted in the cylinder to take up different pipetting volume. Accordingly, the piston stroke is adjusted by setting the piston initially to a predetermined position.
It is a drawback of the pipette device of this type that there is a play between the piston and the cylinder and that this space is filled with residual air. This is detrimental to the pipetting accuracy, in particular under the aspect that the liquids to be handled contain gases or the air drawn in has the same temperature. However, for pipetting a larger volume, an enlarged cylinder chamber is necessary.
A pipetting device of the type above referred to is shown by German specification 1 952 774. The piston of this pipette device comprises a projection and an annular seal cooperating with the cylinder wall. The projection is provided with a fluorocarbon sleeve which is cemented thereon or tightly pressed on. This is an expensive manufacturing step and relative changes in position of the sleeve and projection cannot be excluded over an extended time of use resulting in decreasing the pipetting accuracy. Furthermore, vibrations or impacts transferred from the actuating button are detrimental to the calibration of the pipetting volume. Apparently the piston cannot be guided in a tight and sealing engagement with the cylinder wall so that the sealing ring additionally Serves to make up for the piston play. However, the wear of the sealing ring is relatively high. Furthermore the material of the piston cannot be suited to different substances to be handled with the pipette which must be replaced as a whole when the wear of the piston becomes excessive.
It is an object of the present invention to provide an improved pipetting device which is easy to make, maintains a high degree of accuracy, provides an improved guiding and sealing to reduce the wear and is suited to handle a broad variety of substances without replacing the pipette device as a whole.